This invention is in the area of rapid prototype, high precision three dimensional stereolithography.
Three dimensional stereolithography is a very recently developed prototyping technology for the rapid production of models for form-fit-and-function testing. The process is a revolutionary approach to the preparation of a wide variety of objects without tooling, with the assistance of computer assisted design (CAD) and computer assisted manufacture (CAM).
As disclosed in U.S. Pat. Nos. 4,575,330 and 4,929,402 to Hull, a CAD file of the desired object is prepared and converted mathematically into stacked cross-sections, or layers. The first layer of the object is scanned with a polymerization initiating source, typically an ultraviolet laser, on the surface of a vat of ethylenically unsaturated monomer, or mixture of monomers. The first layer of the model, that is positioned on an elevator platform in the vat, is then lowered a programmed amount with an actuator mechanism, so that a new coating of polymerizable liquid covers the solidified layer. A wiper blade perfects the coating depth, and then the laser draws a new layer on top of the preceding one. This procedure is repeated until the desired three dimensional structure is completed. Webbings can be added to the design as necessary to keep object protrusions from floating away. The prepared object (green body) is a partially cured structure. After removal from the vat, the green body is cured, and sanded, or otherwise smoothed, as necessary.
The '330 patent (see FIG. 4 of that patent) teaches, as an alternative embodiment, floating the ultraviolet-curable liquid on a heavier, immiscible, ultraviolet transparent liquid layer in the vat. In this embodiment, the UV source radiates from below the vat through the ultraviolet transparent material, and is focused at the interface of the two liquids. The object is pulled up out of the ultraviolet-curable liquid, rather than down and further into the liquid, as shown in FIG. 3. of '330. This embodiment is useful to minimize the amount of curable material used. However, the incompletely polymerized greenbody may experience sagging and distortion.
U.S. Pat. No. 5,011,635 to Murphy, et al., provides an apparatus for 3D-stereolithography that includes a fluid phase, a substantially impermeable, movable membrane positioned on top of the fluid phase, a radiation-polymerizable liquid organic phase positioned on top of the membrane and a radiation source positioned above the organic phase. This system also allows a reduction in the volume of polymerizable vat liquid needed in the apparatus. The presence of the membrane, however, adds complex material selection criteria.
U.S. Pat. No. 4,844,144 to Murphy discloses a method of investment casting using a model prepared by 3D-stereolithography, that includes using a polymer precursor fluid in the prototyping that includes an ethylenically unsaturated liquid material that is mixed with an inert low thermoplastic material that weakens the pattern when heated in the investment casting process to prevent thermal expansion of the pattern from cracking the mold. Weakening of parts may exacerbate distortion, leading to inexact finished objects.
The main problem associated with the use of 3D-stereolithography for prototyping is the lack of precise dimensional tolerance. One form of stress that causes distortion develops when material that is being converted from liquid to solid comes into contact with and bonds to previously cured material. This stress can result in curl distortion, wherein individual layers separate from the structure.
Another type of stress occurs when an incompletely polymerized object is annealed (cured by additional heat or blanket radiation, or both simultaneously), because the continuing reaction causes shrinkage of the precisely modeled part. Further, the high temperature needed for curing in the absence of radiation adversely affects the object. If the temperature of cure is too high, the object can soften, further losing its shape.
The extent of dimensional distortion is a function of the exact geometry and spatial design of the object, and the ability of the object to withstand stress, and will vary at different locations on the object. Presently, 3D-stereolithography techniques are limited in exactness to the order of a few thousandths of an inch, even with the use of sophisticated computer algorithms that predict and attempt to compensate for this shrinkage. Further, while post cure warpage may be decreased by increasing the percentage of vat cure, curl distortion increases dramatically as the vat cure reaches completion, due to a buildup of internal stress accompanying successive layer deposition under ambient pressure.
An attempt to solve the problem of post cure distortion is disclosed in U.S. Pat. No. 4,942,001 to Murphy, et al, that utilizes a vat solution that includes from 20 to 80 percent of a resinous polyacrylate or polymethacrylate dissolved in a combination of 10 to 45 weight percent of a liquid polyacrylate or polymethacrylate, which is preferably trifunctional, and 10 to 45 weight percent of N-vinyl monomer. The solution, on curing, provides a lightly cross-linked, solvent swellable, polymeric, thin walled element constituted by heat-softenable solid polymer. The addition of resinous polymers with monomers may increase the viscosity of the polymerizable mixture, slowing down the fluid movement and aggravating curl distortion.
U.S. Pat. No. 4,945,032 to Murphy, et al., discloses that post cure distortion can be reduced by stopping the exposure at any portion of the surface in the formation of the layer and then repeating the exposure at least once again in the production of each surface layer so that the strength and solvent resistance of the formed object are increased. The ultraviolet exposure of each surface layer is preferably carried out as a series of rapid repeated scans of a computer focused laser.
U.S. Pat. No. 4,972,006 to Murphy, et al., discloses that the green body can be cured by immersing it in an aqueous solution bath that includes a water soluble free radical catalyst that is absorbed by the green body. The bath is heated to complete the cure. Although additional cure can be accomplished by the catalyst in the aqueous solution, this approach does not improve the residual warpage problem caused in the post cure step.
U.S. Pat. Nos. 4,999,143 and 5,059,359 to Hull, et al., disclose that curl and distortion can be reduced by, among other things, defining the object in a way to provide built-in supports for the object (webs), and by dividing the surface of the solid model into triangles (PHIGS) using CAD, for better surface resolution. This mechanistic-based approach, while useful, leads to unnecessary and unwanted webs and supports, which must be trimmed away.
U.S. Pat. No. 5,015,424 to Smalley teaches that distortion can be reduced by isolating sections of an object so that stress cannot be transmitted from one section to another. Layer sections prone to curling are isolated by designing small holes or gaps at stress points in the CAD design or the part. These gaps are called "smalleys." Smalleys are also used to reduce birdsnesting (unsecured boundaries in the object that move up and down during manufacture, and give a rough surface finish to the object). This mechanistic-based approach similarly introduces unnecessary complications.
U.S. Pat. Nos. 5,076,974 and 5,164,128 describes a new part building technique called "Weave", which improves dimensional tolerances. Typical x-y cross-hatching methods produce a rather fragile matrix of thin-walled chambers that trap liquid or semi-cured resin inside in much the same way water is trapped in partially frozen ice cubes in a freezer tray.
Post-cure warpage is reduced and surface finish improved as a greater portion of the liquid resin is cured in the vat. Post-cure distortion decreases, in part, because there is less post-cure shrinkage. However, curl distortion resulting from the separation of layers from the structure dramatically increases as the extent of beam cross-hatching is increased and the degree of polymerization approaches 100%, due to the buildup of internal stress.
U.S. Pat. Nos. 5,139,338 and 5,157,423, assigned to Cubital Ltd. disclose a method to prepare three dimensional objects stereolithographically that employs a flood UV curing process as a means to eliminate problems associated with post-curing. Like all rapid prototyping processes, a solid or surface CAD model is first sliced into thin cross sections. A slice is then transferred from the computer to the mask generator, which operates like a photocopier: a negative image of the cross section is produced on a glass mask plate by charging portions of the surface and "developing" the electrostatic image with toner powder. Simultaneously, a thin layer of liquid photopolymer is spread across the surface of the workbench. The mask plate with the negative image of the crosssectional slice is then positioned over the workbench. A shutter above both the mask and the workbench opens for two seconds, allowing strong UV light from a 2-kilowatt lamp to solidify the exposed photopolymer layer all at once. Areas external to the model are left in liquid form.
The exposed mask is then physically wiped down and electrostatically discharged, erasing the mask plate and preparing it for the next negative cross-section image. At the same time, the uncured polymer is removed from the workbench by the combination of forced air and vacuum pressure and is collected for reuse. The workbench moves to the next station, where hot wax is laid down to fill the cavities left by the uncured polymer. At the next station, a cooling plate is applied to solidify the wax, which acts as a support structure to reduce distortion due to gravitational or shrinkage effects. Finally, the surface of the entire polymer/wax layer is milled with a cutter to the desired thickness, which makes the workpiece surface ready to accept the next polymer layer. The steps are repeated until the part is completed. After the model is constructed, the supporting wax is removed with microwave energy and hot air from a blower, and rinsed with solvent. Because each layer is fully cured, no post-curing is required. Although this process can be used to make high precision parts, the parts still exhibit some distortion due to buildup of stress between layers during polymerization.
U.S. Pat. Nos. 4,752,498 and 4,801,477 describe a method for forming three dimensional objects stereolithographically, in which a sufficiently rigid transparent plate or film is placed in contact with a liquid polymer precursor fluid to hold the fluid in a desired shape, and preferably exclude air from the reaction vat. The plate is not sealed on the vat so that volume changes in the vat are made up by the unrestricted supply of fluid from around the irradiated area. It is further suggested that the surface of the transparent plate be made of a material that leaves the irradiated polymer surface capable of further crosslinking so that when a subsequent layer is formed it will adhere thereto. The patents teach that the plate should be made of or contain in its molecules oxygen, copper or other inhibitors to aid in the release of the layer without distorting the solidified photopolymer.
The highest precision obtainable theoretically using the technique of three dimensional stereolithography is the diffraction limit of light (submicron). While the abovedescribed techniques have been used to reduce the distortion of objects made by 3D-stereolithography, fine precision has not yet been attained. There remains a need to provide a method to produce form-fit-and-function models by 3D-stereolithography that provides improved precision.
High precision is necessary in the production of micro and ministructures for use in microelectronics that have high aspect ratio and significant structural height. Micro and ministructures are typically prepared by optical lithography, which has been perfected to attain the 0.5 .mu.m critical dimension (CD) that is necessary for the fabrication of 16 Mbit memory chips. This technology has been modified to make microsensors and microactuators by either bulk or micromachining on silicon wafers.
High aspect ratio microstructures have also been prepared using x-ray lithography with high quantum energy synchrotron radiation. The "LIGA" process (see Becker, et al., Microelectronic Engineering 4 (1986) 35-56, and U.S. Pat. No. 4,990,827) produces microstructures with lateral dimensions in the micrometer range and structural heights of several hundred micrometers. The LIGA process is schematically illustrated in FIG. 1. A polymeric material (resist) which changes its dissolution rate in a liquid solvent (developer) on high energy irradiation, is exposed through an x-ray mask to highly intense parallel x-rays. The radiation source is an electron synchrotron or an electron storage ring that can generate the highly collimated photon flux in the spectral range required for precise deep-etch x-ray lithography in thick resist layers. As an example, a pattern thickness between 10 and 1,000 .mu.m typically requires an optimal critical wavelength of synchrotron radiation of from 0.1 and 1 nm. In the next step, the resist structure is used as a template in an electroforming process in which metal is deposited onto the electrically conductive substrate (galvanoformation). The polymeric resist is then removed to provide a highly precise metal mold. The secondary plastic mold is prepared by introducing a polymeric mold material into the metal mold cavities through the holes of a gate plate. The plate has a formlocking connection with the polymeric microstructure, and after hardening of the molding resin the plate serves as an electrode in a second electroforming process for generating secondary metallic microstructures. The LIGA process produces highly precise secondary structures, including those with an aspect ratio of up to 100 and minimum lateral dimensions in the micrometer range.
The LIGA process has been used to produce microsensors, measuring devices for vibration and acceleration, microoptical devices and symmetry, fluidic devices, and electrical and optical microconnectors. Primary disadvantages associated with the LIGA process are that it can only produce fully attached metal structures, and that the process requires the use of an electron syncrotron, that is not readily available.
Guckel et. al. (Proceedings of International Conference on Solid-State Sensors and Actuators, 1991) reported a new process called sacrificial LIGA (SL1GA). The process is illustrated in FIG. 2. The addition of a sacrificial layer to the LIGA process facilitates the fabrication of fully attached, partially attached or completely free metal structures. Because device thicknesses are typically larger than 10 .mu.m and smaller than 300 .mu.m, freestanding structures do not distort geometrically if reasonable strain control of the plated film is achieved. However, the process still requires the use of an electron syncrotron, that is not readily available. It would be useful to provide a process and apparatus for the production of high aspect ratio micro and ministructures for microelectronics that do not require the use of an electron syncrotron.
Therefore, it is an object of the present invention to provide a method for the preparation of objects by 3D-stereolithography that minimizes object distortion.
It is an additional object of the present invention to broaden the selection of polymer precursors to include slow-reacting systems and to include particle-containing fluids that upon solidification form real parts that possess dual polymer and ceramic properties and/or magnetic, electrical, or optical attributes.
It is yet another object of the present invention to accomplish precision polymerization within a short time, so that real parts can be generated quickly.
It is still another object of the present invention to provide a process and apparatus for the production of high aspect ratio micro and ministructures for microelectronics.